Secondary batteries are rechargeable batteries including Ni—Cd batteries, Ni—MH batteries, and lithium ion batteries. Recently, the lithium ion batteries having energy density higher than that of the Ni—Cd batteries or the Ni—MH batteries have been extensively used. The lithium ion battery can be fabricated in a compact size with a light weight, so the lithium ion battery can be effectively utilized as a power source for portable electronic appliances, such as portable phones, camcorders or notebook computers. In addition, the lithium ion battery is extensively used as a power source for an electric vehicle, so the lithium ion battery has been currently spotlighted as a next-generation energy storage medium.
However, the secondary battery, in particular, the lithium ion battery is vulnerable to overcharge. Such an overcharge may cause accidental ignition or explosion of the lithium ion battery, thereby incurring a dangerous accident or property loss. Therefore, it is necessary to prevent or restrict the overcharge of the secondary battery or to solve problems derived from the overcharge of the secondary battery.
For instance, when the lithium ion battery is subject to the overcharge, a side reaction may increasingly occur between a cathode active material and an electrolyte of the lithium ion battery. Such side reaction destroys the structure of the cathode active material while causing an oxidation reaction of the electrolyte. In the meantime, lithium may be deposited on an anode active material consisting of graphite, etc. If the voltage of the secondary battery rises in a state in which the secondary battery has been overcharged, accidental ignition or explosion of the secondary battery may occur.
The above problem may become serious if the secondary battery is used for a high voltage power source. For instance, if the lithium ion secondary battery is charged by the power source of a vehicle through cigar jack, 12V is applied in cases of automobiles, and 24V is applied in cases of freight cars by connecting two power sources of 12V in series. In this case, if an excessive voltage deviating from the standard for the secondary battery is suddenly applied to the secondary battery, a dangerous accident may occur, so that it is necessary to provide a safety device capable of effectively protecting the secondary battery from the excessive voltage.
In the meantime, recently, there are great demands for safety devices that can be fabricated with a simple structure at a low cost. That is, it is necessary to provide an inexpensive safety device having a simple structure than that of a conventional safety device equipped with a protective circuit, such as a PTC circuit.
Currently, structural characteristics of an electrolyte, a separator film and an electrode of the secondary battery have been improved, so an electrolyte and electrode assembly is provided with a safety means capable of preventing the secondary battery from being overcharged. Thus, there are various attempts to fabricate the secondary battery by using a bare cell only without employing a protective circuit in order to reduce the manufacturing cost of the secondary battery. However, even in this case, a basic safety means should be provided in the secondary battery to prevent the overcharge of the secondary battery or accidents derived from the overcharge of the secondary battery.
For instance, Japanese Patent Unexamined Publication Nos. 5-325943 and 2003-284237, and U.S. Pat. No. 6,331,763 disclose a zener diode and a thermal fuse as a safety device for the secondary battery. According to the above conventional technologies, two thermal fuses, which are connected to each other in series, and one zener diode are inserted into the secondary battery.
In this case, the current may flow towards the zener diode other than the battery cell when the secondary battery is subject to the overcharge, so that the thermal fuse connected to the zener diode is cut off due to heat derived from the excessive current applied to the zener diode, thereby shutting off the current being applied to the secondary battery. Japanese Patent Unexamined Publication Nos. 5-325943 and 2003-284237 disclose that it is preferred to use a zener diode having a breakdown voltage (zener voltage) similar to or higher than a maximum charge voltage of the battery. In addition, U.S. Pat. No. 6,331,763 discloses a zener diode having a breakdown voltage lower than the maximum charge voltage of the battery.
The battery or the cell can be prevented from being overcharged if the breakdown voltage of the zener diode is lower than the charge voltage of the battery or if the breakdown voltage of the zener diode is identical to or slightly higher than the charge voltage of the battery. However, in this case, the zener diode may have the leakage current. That is, the above conventional technologies cannot solve the problem of self-discharge of the battery caused by the leakage current of the zener diode.
It is generally known in the art that the zener diode has the leakage current under a predetermined voltage lower than the breakdown voltage of the zener diode by at least 1V. Thus, in a case of the battery, the zener diode inevitably causes the leakage current in the range of the operating voltage of the battery (less than 4.2V). If the leakage current occurs at a device connected to a cathode and an anode of the battery, the battery may be self-discharged, so that the operating time of the battery may be reduced after the battery has been charged and life span of the battery may be shortened. Therefore, according to the above conventional technologies, current reduction may always occur in the battery.
That is, if the leakage current occurs at the device connected between the cathode and the anode of the battery, the battery is self-discharged so that the life span of the battery may be shortened. Thus, if the zener diode is attached between the cathode and the anode of the battery, current reduction may always occur in the battery. If a zener diode, which does not cause the leakage current in the charge voltage of the battery, is used for the battery, the current cannot be sufficiently discharged when the battery is subject to the overcharge. In addition, when a high current is applied to the zener diode, the zener diode is broken so that the zener diode may not play its original role and if the voltage rises, the resistance also increases so that the current is interrupted. In the meantime, according to the above conventional technologies, the current is shut off by means of the thermal fuse when the zener diode is subject to overheat. However, in this case, it is difficult to prevent the leakage current of the zener diode under the charge voltage (4.2V-4.5V) of the battery because the thermal fuse should operate at the temperature above 60° C., which is a normal operating temperature of the battery, and a voltage difference of at least 0.5V is necessary in order to allow the zener diode suitable for the current of 50 to 200 mA, a normal charge current of a secondary battery generally used, to reach the temperature of 60° C. In particular, the above zener diode, which does not cause the leakage current under the charge voltage (4.2V-4.5V) of the battery, is not adaptable when it is necessary to apply the current of 50 to 200 mA to the zener diode without causing the overcharge of the battery. Although the above conventional technologies disclose the charge/discharge cycle for the battery, they do not suggest the solution for the self-discharge of the battery caused by the leakage current of the zener diode.